comment.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * comment.c
 *
 * PostgreSQL object comments utility code.
 *
 * Copyright (c) 1996-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/commands/comment.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/genam.h"
#include "access/htup_details.h"
#include "access/relation.h"
#include "access/table.h"
#include "catalog/indexing.h"
#include "catalog/objectaddress.h"
#include "catalog/pg_database.h"
#include "catalog/pg_description.h"
#include "catalog/pg_shdescription.h"
#include "commands/comment.h"
#include "miscadmin.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/rel.h"
 
 
/*
 * CommentObject --
 *
 * This routine is used to add the associated comment into
 * pg_description for the object specified by the given SQL command.
 */
ObjectAddress
CommentObject(CommentStmt *stmt)
{
    Relation    relation;
    ObjectAddress address = InvalidObjectAddress;
    bool        missing_ok;
 
    /*
     * When loading a dump, we may see a COMMENT ON DATABASE for the old name
     * of the database.  Erroring out would prevent pg_restore from completing
     * (which is really pg_restore's fault, but for now we will work around
     * the problem here).  Consensus is that the best fix is to treat wrong
     * database name as a WARNING not an ERROR; hence, the following special
     * case.
     */
    if (stmt->objtype == OBJECT_DATABASE)
    {
        char       *database = strVal(stmt->object);
 
        if (!OidIsValid(get_database_oid(database, true)))
        {
            ereport(WARNING,
                    (errcode(ERRCODE_UNDEFINED_DATABASE),
                     errmsg("database \"%s\" does not exist", database)));
            return address;
        }
    }
 
    /*
     * During binary upgrade, allow nonexistent large objects so that we don't
     * have to create them during schema restoration.  pg_upgrade will
     * transfer the contents of pg_largeobject_metadata via COPY or by
     * copying/linking its files from the old cluster later on.
     */
    missing_ok = IsBinaryUpgrade && stmt->objtype == OBJECT_LARGEOBJECT;
 
    /*
     * Translate the parser representation that identifies this object into an
     * ObjectAddress.  get_object_address() will throw an error if the object
     * does not exist, and will also acquire a lock on the target to guard
     * against concurrent DROP operations.
     */
    address = get_object_address(stmt->objtype, stmt->object,
                                 &relation, ShareUpdateExclusiveLock,
                                 missing_ok);
 
    /* Require ownership of the target object. */
    check_object_ownership(GetUserId(), stmt->objtype, address,
                           stmt->object, relation);
 
    /* Perform other integrity checks as needed. */
    switch (stmt->objtype)
    {
        case OBJECT_COLUMN:
 
            /*
             * Allow comments only on columns of tables, views, materialized
             * views, composite types, and foreign tables (which are the only
             * relkinds for which pg_dump will dump per-column comments).  In
             * particular we wish to disallow comments on index columns,
             * because the naming of an index's columns may change across PG
             * versions, so dumping per-column comments could create reload
             * failures.
             */
            if (relation->rd_rel->relkind != RELKIND_RELATION &&
                relation->rd_rel->relkind != RELKIND_VIEW &&
                relation->rd_rel->relkind != RELKIND_MATVIEW &&
                relation->rd_rel->relkind != RELKIND_COMPOSITE_TYPE &&
                relation->rd_rel->relkind != RELKIND_FOREIGN_TABLE &&
                relation->rd_rel->relkind != RELKIND_PARTITIONED_TABLE)
                ereport(ERROR,
                        (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                         errmsg("cannot set comment on relation \"%s\"",
                                RelationGetRelationName(relation)),
                         errdetail_relkind_not_supported(relation->rd_rel->relkind)));
            break;
        default:
            break;
    }
 
    /*
     * Databases, tablespaces, and roles are cluster-wide objects, so any
     * comments on those objects are recorded in the shared pg_shdescription
     * catalog.  Comments on all other objects are recorded in pg_description.
     */
    if (stmt->objtype == OBJECT_DATABASE || stmt->objtype == OBJECT_TABLESPACE
        || stmt->objtype == OBJECT_ROLE)
        CreateSharedComments(address.objectId, address.classId, stmt->comment);
    else
        CreateComments(address.objectId, address.classId, address.objectSubId,
                       stmt->comment);
 
    /*
     * If get_object_address() opened the relation for us, we close it to keep
     * the reference count correct - but we retain any locks acquired by
     * get_object_address() until commit time, to guard against concurrent
     * activity.
     */
    if (relation != NULL)
        relation_close(relation, NoLock);
 
    return address;
}
 
/*
 * CreateComments --
 *
 * Create a comment for the specified object descriptor.  Inserts a new
 * pg_description tuple, or replaces an existing one with the same key.
 *
 * If the comment given is null or an empty string, instead delete any
 * existing comment for the specified key.
 */
void
CreateComments(Oid oid, Oid classoid, int32 subid, const char *comment)
{
    Relation    description;
    ScanKeyData skey[3];
    SysScanDesc sd;
    HeapTuple   oldtuple;
    HeapTuple   newtuple = NULL;
    Datum       values[Natts_pg_description];
    bool        nulls[Natts_pg_description];
    bool        replaces[Natts_pg_description];
    int         i;
 
    /* Reduce empty-string to NULL case */
    if (comment != NULL && strlen(comment) == 0)
        comment = NULL;
 
    /* Prepare to form or update a tuple, if necessary */
    if (comment != NULL)
    {
        for (i = 0; i < Natts_pg_description; i++)
        {
            nulls[i] = false;
            replaces[i] = true;
        }
        values[Anum_pg_description_objoid - 1] = ObjectIdGetDatum(oid);
        values[Anum_pg_description_classoid - 1] = ObjectIdGetDatum(classoid);
        values[Anum_pg_description_objsubid - 1] = Int32GetDatum(subid);
        values[Anum_pg_description_description - 1] = CStringGetTextDatum(comment);
    }
 
    /* Use the index to search for a matching old tuple */
 
    ScanKeyInit(&skey[0],
                Anum_pg_description_objoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(oid));
    ScanKeyInit(&skey[1],
                Anum_pg_description_classoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(classoid));
    ScanKeyInit(&skey[2],
                Anum_pg_description_objsubid,
                BTEqualStrategyNumber, F_INT4EQ,
                Int32GetDatum(subid));
 
    description = table_open(DescriptionRelationId, RowExclusiveLock);
 
    sd = systable_beginscan(description, DescriptionObjIndexId, true,
                            NULL, 3, skey);
 
    while ((oldtuple = systable_getnext(sd)) != NULL)
    {
        /* Found the old tuple, so delete or update it */
 
        if (comment == NULL)
            CatalogTupleDelete(description, &oldtuple->t_self);
        else
        {
            newtuple = heap_modify_tuple(oldtuple, RelationGetDescr(description), values,
                                         nulls, replaces);
            CatalogTupleUpdate(description, &oldtuple->t_self, newtuple);
        }
 
        break;                  /* Assume there can be only one match */
    }
 
    systable_endscan(sd);
 
    /* If we didn't find an old tuple, insert a new one */
 
    if (newtuple == NULL && comment != NULL)
    {
        newtuple = heap_form_tuple(RelationGetDescr(description),
                                   values, nulls);
        CatalogTupleInsert(description, newtuple);
    }
 
    if (newtuple != NULL)
        heap_freetuple(newtuple);
 
    /* Done */
 
    table_close(description, NoLock);
}
 
/*
 * CreateSharedComments --
 *
 * Create a comment for the specified shared object descriptor.  Inserts a
 * new pg_shdescription tuple, or replaces an existing one with the same key.
 *
 * If the comment given is null or an empty string, instead delete any
 * existing comment for the specified key.
 */
void
CreateSharedComments(Oid oid, Oid classoid, const char *comment)
{
    Relation    shdescription;
    ScanKeyData skey[2];
    SysScanDesc sd;
    HeapTuple   oldtuple;
    HeapTuple   newtuple = NULL;
    Datum       values[Natts_pg_shdescription];
    bool        nulls[Natts_pg_shdescription];
    bool        replaces[Natts_pg_shdescription];
    int         i;
 
    /* Reduce empty-string to NULL case */
    if (comment != NULL && strlen(comment) == 0)
        comment = NULL;
 
    /* Prepare to form or update a tuple, if necessary */
    if (comment != NULL)
    {
        for (i = 0; i < Natts_pg_shdescription; i++)
        {
            nulls[i] = false;
            replaces[i] = true;
        }
        values[Anum_pg_shdescription_objoid - 1] = ObjectIdGetDatum(oid);
        values[Anum_pg_shdescription_classoid - 1] = ObjectIdGetDatum(classoid);
        values[Anum_pg_shdescription_description - 1] = CStringGetTextDatum(comment);
    }
 
    /* Use the index to search for a matching old tuple */
 
    ScanKeyInit(&skey[0],
                Anum_pg_shdescription_objoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(oid));
    ScanKeyInit(&skey[1],
                Anum_pg_shdescription_classoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(classoid));
 
    shdescription = table_open(SharedDescriptionRelationId, RowExclusiveLock);
 
    sd = systable_beginscan(shdescription, SharedDescriptionObjIndexId, true,
                            NULL, 2, skey);
 
    while ((oldtuple = systable_getnext(sd)) != NULL)
    {
        /* Found the old tuple, so delete or update it */
 
        if (comment == NULL)
            CatalogTupleDelete(shdescription, &oldtuple->t_self);
        else
        {
            newtuple = heap_modify_tuple(oldtuple, RelationGetDescr(shdescription),
                                         values, nulls, replaces);
            CatalogTupleUpdate(shdescription, &oldtuple->t_self, newtuple);
        }
 
        break;                  /* Assume there can be only one match */
    }
 
    systable_endscan(sd);
 
    /* If we didn't find an old tuple, insert a new one */
 
    if (newtuple == NULL && comment != NULL)
    {
        newtuple = heap_form_tuple(RelationGetDescr(shdescription),
                                   values, nulls);
        CatalogTupleInsert(shdescription, newtuple);
    }
 
    if (newtuple != NULL)
        heap_freetuple(newtuple);
 
    /* Done */
 
    table_close(shdescription, NoLock);
}
 
/*
 * DeleteComments -- remove comments for an object
 *
 * If subid is nonzero then only comments matching it will be removed.
 * If subid is zero, all comments matching the oid/classoid will be removed
 * (this corresponds to deleting a whole object).
 */
void
DeleteComments(Oid oid, Oid classoid, int32 subid)
{
    Relation    description;
    ScanKeyData skey[3];
    int         nkeys;
    SysScanDesc sd;
    HeapTuple   oldtuple;
 
    /* Use the index to search for all matching old tuples */
 
    ScanKeyInit(&skey[0],
                Anum_pg_description_objoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(oid));
    ScanKeyInit(&skey[1],
                Anum_pg_description_classoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(classoid));
 
    if (subid != 0)
    {
        ScanKeyInit(&skey[2],
                    Anum_pg_description_objsubid,
                    BTEqualStrategyNumber, F_INT4EQ,
                    Int32GetDatum(subid));
        nkeys = 3;
    }
    else
        nkeys = 2;
 
    description = table_open(DescriptionRelationId, RowExclusiveLock);
 
    sd = systable_beginscan(description, DescriptionObjIndexId, true,
                            NULL, nkeys, skey);
 
    while ((oldtuple = systable_getnext(sd)) != NULL)
        CatalogTupleDelete(description, &oldtuple->t_self);
 
    /* Done */
 
    systable_endscan(sd);
    table_close(description, RowExclusiveLock);
}
 
/*
 * DeleteSharedComments -- remove comments for a shared object
 */
void
DeleteSharedComments(Oid oid, Oid classoid)
{
    Relation    shdescription;
    ScanKeyData skey[2];
    SysScanDesc sd;
    HeapTuple   oldtuple;
 
    /* Use the index to search for all matching old tuples */
 
    ScanKeyInit(&skey[0],
                Anum_pg_shdescription_objoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(oid));
    ScanKeyInit(&skey[1],
                Anum_pg_shdescription_classoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(classoid));
 
    shdescription = table_open(SharedDescriptionRelationId, RowExclusiveLock);
 
    sd = systable_beginscan(shdescription, SharedDescriptionObjIndexId, true,
                            NULL, 2, skey);
 
    while ((oldtuple = systable_getnext(sd)) != NULL)
        CatalogTupleDelete(shdescription, &oldtuple->t_self);
 
    /* Done */
 
    systable_endscan(sd);
    table_close(shdescription, RowExclusiveLock);
}
 
/*
 * GetComment -- get the comment for an object, or null if not found.
 */
char *
GetComment(Oid oid, Oid classoid, int32 subid)
{
    Relation    description;
    ScanKeyData skey[3];
    SysScanDesc sd;
    TupleDesc   tupdesc;
    HeapTuple   tuple;
    char       *comment;
 
    /* Use the index to search for a matching old tuple */
 
    ScanKeyInit(&skey[0],
                Anum_pg_description_objoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(oid));
    ScanKeyInit(&skey[1],
                Anum_pg_description_classoid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(classoid));
    ScanKeyInit(&skey[2],
                Anum_pg_description_objsubid,
                BTEqualStrategyNumber, F_INT4EQ,
                Int32GetDatum(subid));
 
    description = table_open(DescriptionRelationId, AccessShareLock);
    tupdesc = RelationGetDescr(description);
 
    sd = systable_beginscan(description, DescriptionObjIndexId, true,
                            NULL, 3, skey);
 
    comment = NULL;
    while ((tuple = systable_getnext(sd)) != NULL)
    {
        Datum       value;
        bool        isnull;
 
        /* Found the tuple, get description field */
        value = heap_getattr(tuple, Anum_pg_description_description, tupdesc, &isnull);
        if (!isnull)
            comment = TextDatumGetCString(value);
        break;                  /* Assume there can be only one match */
    }
 
    systable_endscan(sd);
 
    /* Done */
    table_close(description, AccessShareLock);
 
    return comment;
}